It is well known that disk refiners comprise two opposed refining disks, one or both of which can be rotatably mounted. A plurality of refiner segments are arranged on these refining disks and are generally formed with a pattern of alternating bars and intermediate grooves therebetween. These refining disks are arranged such that the refiner segments form a refining gap, through which the fibrous material is passed so that it can be disintegrated by the bars on the individual segments.
The bars and grooves of these refiner segments normally extend substantially radially, but can also be arranged more or less obliquely in relation to the radius of the segments. The pattern of these bars and grooves can also be divided into zones located outwardly circumferentially from each other and having different types of bars thereon in such zones.
It has been found to be advantageous, especially during disintegration of fibrous material having a high concentration, to place flow restrictions, such as so-called "dams," across the grooves in the refiner segments in order to prevent untreated material from passing outwardly through the refining gaps. These dams tend to force the material upwardly out of the grooves, so that it can then be subjected to treatment between the bars on the refiner segments on opposing refining disks.
In those plants where the fibrous raw material contains significant amounts of sand and other impurities, the bars on the refiner segments can be worn out prematurely. This takes place, in particular, in plants where fiberboard is being manufactured.
When the refiner segments are provided with dams, the wear generally concentrates on the bars about these dams. It has also been found that the dams are not worn to the same degree as the bars about them, and the dams will therefore project above the bars on the refiner segments, which can result in their coming into contact with and wearing down the bars on the refiner segments on the opposed refining disks. The refiner segments are thus subjected to much stronger wear, resulting in an unacceptably short service life for the refiner segments.
In order to overcome these problems, it has become necessary in certain cases to install rather expensive equipment, for the removal of the sand or other material prior to the disk refiner to the greatest possible extent. Alternatively, refiner segments have been used without dams. In these cases, however, additional steps must be taken to deal with the reduced quality of the finally distributed material, such as by refining in several steps.